In operation, overload couplings or clutches of the kind considered here tend to slip when overloaded, that is to slip when the torque is increased to an impermissible great extent. Overload clutches of this kind are provided for the protection of machines or machine parts particularly when a powerful driving unit must work against considerable counterforces; if locking occurs in the working machine as the result of a breakdown, either the working machine may be destroyed through overload or the driving unit may be damaged by running at high speed without load. This problem occurs particularly in screw machines such as extruder machines, in which the circuit arrangement of the invention finds its preferred application.
Although commercial slip indicators exist, which in principle are also suitable for the task of monitoring overload clutches, it has nevertheless been found that these known devices either react to a deviation from zero slip or else respond only at comparatively high slip values. Both of these response limits are unsuitable for the preferred application of the invention. It has in fact been found that overload clutches suitable for extruder machines have a continuous slight operating slip which leads to a kind of self-cleaning effect to which the accurate and reliable response of the overload clutch when subjected to overload is attributable. If they were adjusted to zero slip the available devices would therefore continuously bring about overload release even during operation under normal conditions, although there would actually be no justification for this, whereas on the other hand the adjustable value of slip other than zero is already so great that because of the powerful driving forces the heat generated in the overload clutch would lead to destruction of the clutch.
It has already been attempted to produce slip monitoring circuits which work with greater sensitivity, these being based on the counting of pulse transmissions from each of the two parts of the clutch; when there is no slip, each pulse transmission from the driving part of the clutch is followed by a pulse tramsmission from the driven part of the clutch which neutralizes this first pulse transmission. When slip occurs, the second, compensating pulse is increasingly retarded until finally another pulse from the driving part of the clutch arrives before the arrival of the compensating pulse, so that the compensating part of the clutch, which acts after the driving part, will compensate only one of the two first pulses. These pulse transmissions are converted into a voltage level, and a subsequent voltage level or voltage difference evaluation initiates the monitoring or check signal for indicating impermissibly high slip. It has however been found that even with slip monitoring circuits of this kind the required sensitivity of response, which has to be asked of the circuit arrangement because of the comparatively low operating slip extending up to about 0.12%, cannot be achieved. One important reason for this is that, because of manufacturing and functional tolerances of these circuit arrangements based on voltage difference measurement, the necessary accuracies cannot be permanently maintained when operating with extremely low permissible slip.